The following publications are representative of the most relevant prior art known to Applicants at the time of the filing of this application.
______________________________________ United States Patents 4,487,644 December 11, 1984 Gupta et al. (I) 4,514,240 April 30, 1985 Heraud 4,526,649 July 2, 1985 Gupta et al. (II) Other Publications EP0209672 January 28, 1987 Gyarmati et al. T. J. Moore, J. Am. Ceram. Soc., C-151-C-153 (1985) --68(6) NASA Tech. Briefs, July/August 1986, pp. 118-119 ______________________________________
Silicon carbide molded bodies having complex shapes are in practice made of several parts which have then been joined together. However, in view of the relatively inert nature of silicon carbide, there has been considerable difficulty in effecting the joining operation, particularly when the end use of the joined silicon carbide pieces will be at temperatures of greater than 1,500 degrees C. Accordingly, the art is replete with the use of various adhesives/glues to be used to join two silicon carbide pieces. Examples of these include various metal alloys, molten silicon, platinum pastes, molybdenum borides, silicon carbide slips, and the like. However, all of these prior art joining methods entail placing some component between the pieces Which means that the resultant joint will inherently be weaker than the SiC pieces and thus the failure point in the finished part. Moreover, none of the prior joining methods has been sufficient to produce a component beta-silicon carbide part which has a tensile strength of 20,000 psi at 1530 C.
The joining of alpha-silicon carbide parts in the absence of any adhesive/glue interlayer has been attempted in the past. This joining is generally referred to as welding. Moore discloses polishing commercial sintered alpha-SiC blocks (theoretical density 98%) with a 320-grit diamond wheel and then attempting joining be hot isostatic pressing at 1950 C. and 138 MPa for 2 hours. When no interlayer was present "absolutely no welding took place", Moore goes on to explain that this unsuccessful result was "not unexpected in view of the fact that SiC powder is known to be difficult to sinter without sintering aids."
NASA Tech. Briefs confirms that under hot isostatic pressing (HIP) conditions of 1950 C. at 20,000 psi for 2 hours with no plastic deformation, SiC could not be welded at all.
U.S. Pat. No. 4,487,644 (Gupta I) discloses polishing silicon carbide surfaces to 1 um, incorporating at least 8 % excess silicon on at least one of the surfaces to be joined, and then heating of the fitted-together bodies in an inert atmosphere under pressure at a temperature of between 1,500 and 1,800 C. and with a force on the bonding surfaces of 1 to 10 kg/cm.sup.2 (14.2 to 142 psi).
The difficulty of joining silicon carbide pieces is such that, two years after Gupta I, Gupta II (U.S. Pat. No. 4,526,649) teaches that the way to do so is to first roughen the silicon carbide surfaces to be joined to a depth of about 100 to 500 um by removal of excess silicon, if present, and then to fill the space with a cokable resin and add liquid silicon to react with the resin at elevated temperatures but with no applied force.
EPO No. 209,672 discloses polishing the surfaces of sintered or hot pressed silicon carbide bodies to be joined, then interposing a maximum 1 um thick coating of at least one metal carbide and/or metal silicide-forming element, and then welding in an inert or reducing atmosphere at elevated temperature under a pressure of 150 to 15,000 psi. The thin coating may be applied by vapour coating or sputtering.
U.S. Pat. No. 4,514,240 discloses the use of an interlayer between two unsintered (porous) silicon carbide pieces, but then goes on to advocate the use of chemical vapor deposition both in the porous bond and in the bonded porous silicon carbide parts to complete the bonding operation.
Accordingly, there is a need for a method of joining beta-silicon carbide pieces with such integrity that the joint is at least as strong as the underlying silicon carbide at temperatures in excess of 1,500 C. In light of this strength at temperature requirement for numerous future silicon carbide applications, there is need for a joining method which will produce a joint which, under scanning electron microscopic examination, is indistinguishable from the pieces being joined. The use of silicon carbide at such elevated temperatures has been severely limited due to the lack of such a joining method. Accordingly, it is an object of the present invention to satisfy these needs and thereby expand the opportunities for using silicon carbide in more complex shapes.